The present invention relates to surgery performed by local heating guided by magnetic resonance (MR) imaging methods of imaging, and more particularly to surgery performed by pulsed local heating guided by magnetic resonance (MR) imaging.
Conventional Magnetic Resonance Imaging (MRI) provides the radiologist with cross sectional views of the anatomy for diagnosis of pathology. MRI provides excellent contrast between different tissues and is useful in planning surgical procedures. A tumor is much more visible in an MR image than as seen in actual surgery because tumor and normal tissues often look similar in surgery. The tumor can also be obscured by blood during surgery. Researchers at Brigham and Womens Hospital, Boston, Mass. have proposed treatment of deep lying tumors by laser surgery. F. A. Jolesz, A. R. Bleire, P. Jakob, P. W. Ruenzel, K. Huttl, G. J. Jako, "MR Imaging of Laser-Tissue Interactions", Radiology 168:249 (1989). Thus, in the case of brain tumors, the patient is first scanned in an MRI system to locate the tumor and plan a safe trajectory between the entry and target points. This can be accomplished by a MRI device employing fast scan apparatus such as U.S. Pat. No. 4,961,054 Gradient Current Speed-up Circuit for High-speed NMR Imaging System by John N. Park, Otward M. Mueller, and Peter B. Roemer, issued Oct. 2, 1990, or U.S. Pat. No. 5,017,871 Gradient Current Speed-up Circuit for High-speed NMR Imaging System, by Otward M. Mueller, and Peter B. Roemer, issued May 21, 1991 both assigned to the present assignee and hereby incorporated by reference. A small burr hole is drilled in the skull and a hollow needle containing an optical fiber is then inserted into the tumor. The patient is then placed back into the MRI system to view the region heated by the laser using a temperature sensitive pulse sequence. Temperature Sensitive pulse sequence is described in U.S. Pat. No. 4,914,608 In-vivo Method for Determining and Imaging Temperature of an Object/Subject from Diffusion Coefficients Obtained by Nuclear Magnetic Resonance, Denis LeBihan, Jose Delannoy, and Ronald L. Levin issued Apr. 3, 1990 and hereby incorporated by reference. Experiments on animals show that a heated zone above a critical temperature destroys tissue. This zone increases in size with time as the heat is applied to reach a steady state or both temperature and heat flow. If the maximum temperature is limited to 100 deg. C, then the laser heated zone, the area exceeding a critical temperature causing destruction of tissue, approaches 1 centimeter in diameter. It is difficult to predict the heated zone geometry because the heat flow depends on the profusion of blood as well as the tissue thermal properties.
Tumors have been selectively destroyed in cancer patients using focussed ultrasound heating at the University of Arizona, B. E. Billard, K. Hynynen and P. B. Roemer Effects of Physical Parameters on High Temperature Ultrasound Hyperthermia Ultrasound in Med. & Biol. Vol. 16, No. 4, pp. 409-420, 1990 hereby incorporated by reference. Billard et al. disclosed that the control of heat was improved by using short laser pulses where the effect of blood perfusion is negligible. However, since they did not image the temperature distribution, it was difficult to hit small, deep laying targets.
It would be beneficial to be able to accurately localize heat to selectively kill or destroy tumor tissue without damage to surrounding healthy tissue.